Why Fish have Gills

With the exception of viruses and anaerobic bacteria, oxygen is required by all living organisms to survive.  Even plants, which for the most part absorb only carbon dioxide, need oxygen at night when there is no light for photosynthesis. Terrestrial animals obtain oxygen by breathing in air from their surrounding atmosphere. These animals have structures called lungs which process the raw atmosphere and distribute oxygen to all parts of their body.

Fish and other aquatic creatures are no exception when it comes to the need for oxygen. However lungs would be unsuitable to fish as they require oxygen to be taken into the body in a gaseous medium such as air which then diffuses into the deoxygenated blood carried past the lungs by the pulmonary vein.

Therefore, instead of using air-breathing structures like their land-dwelling counterparts, fish’s respiratory organs, gills, are built for extracting oxygen from water and then excreting carbon dioxide. The gill slits of fish are thought to be the evolutionary ancestors of structures found in mammals such as the tonsils and the thymus gland which evolved when aquatic organisms crawled onto land and began to breathe air. Fish, however, which still live in water, retain their ancestral breathing structures; they are essential to the fish’s continued survival. As they cannot live outside of water, they thus must be able to extract everything they need from an aquatic environment.

Although there are many different types of gills, the general principle remains the same. The typical bony fish will have five pairs of gills on either side of its head, each lined with tiny capillaries for the temporary storage of blood. The gills are vent-like in appearance, taking the form of tiny flaps and consisting of slender filaments of tissue, branches and cartilage. They are covered by a thin, bony structure called the operculum. The gill tissue is folded many times in order to maximize surface area. This is essential to aquatic respiration as water contains only a twentieth of the dissolved oxygen present in air.

To begin the process of respiration, the fish swallows a mouthful of water, keeping it from escaping again by use of mouth valves. This water then passes over the gills and out of the fish again beneath the operculum. Before it escapes, however, oxygen diffuses from the water into the deoxygenated blood stored in the capillaries through the thin gill tissue. This oxygen is then carried by the blood to all parts of the body while carbon dioxide is expelled by the same process, but in reverse.

Gills also serve a variety of other functions besides the respiratory process. They are used by bony fish, specifically teleosts , to excrete electrolytes and thus regulate the osmolarity of their internal fluids. They also aid fish in the expelling of nitrogenous waste as well as regulate the pH or acidity in the fish’s body.

It is clear then that gills are essential to the survival of fish; not only do they allow breathing underwater but they also form part of other important processes in a fish’s body.


Andrews, Chris; Adrian Exell, Neville Carrington (2003). Manual Of Fish Health. Firefly Books.

Wienberger, Cockrill, Mandel. Principles of Pulmonary Medicine. Elsevier Science.

Fish Facts – National Geographic